of energy have been recognized since Galileo's time, (and 

 even matter itself is now considered to be a kind of energy), 

 this principle of the conservation of energy within any 

 isolated system still holds true, and this is one of the 

 fundamental laws of Nature. Energy, although essentially 

 intangible, is concerned in all changes taking place in 

 the Universe, and is the other fundamental constituent of 

 the Universe; (or the only one, if matter is regarded as 

 a kind of energy). There is, today, no definite proof of 

 the existence of any entities other than matter and energy, 

 but a few rare human psychological phenomena appear 

 to suggest that something else may possibly exist. 



During the centuries following the first studies of 

 matter in motion, the concepts of matter and energy 

 became clearer as further discoveries were made in physics 

 and chemistry. The sixteenth and seventeenth centuries 

 saw the experimental proof of the constancy of total 

 mass (as measured by the weight) of substances taking 

 part in chemical reactions — i.e. a chemical reaction 

 involved only the formation of new substances from old, 

 without any change in mass. Gases were realized to be 

 also a form of matter and to have appreciable weight; 

 owing to their very small density they had previously 

 escaped notice to a large extent. They were found to be 

 involved in many important chemical reactions, including 

 the combustion of inflammable materials and in the res- 

 piration of animals. As for reactions of non-living sub- 

 stances, it was also shown that the total mass of a living 

 thing changed only by an amount equal to the mass of any 

 materials taken in or given out by the animal: in par- 

 ticular, respiration was proved to be a process of com- 

 bustion involving the oxygen from the air, but here no 

 high temperature was produced as in the flame. The law 



48 



